Recently, in place of iron material, aluminum alloy has been increasingly employed for producing suspension parts for vehicles in order to reduce the weight of the parts. Particularly, the suspension parts for vehicles have been produced through forging in order to enhance their mechanical strength and to reduce the amount of raw material employed for producing a product. Examples of the parts employed in a vehicle suspension include an upper arm and a lower arm.
Since an upper arm 54 shown in FIG. 5, which is a suspension part for a vehicle, has branches 51, 52 and 53 extending in three directions, difficulty is encountered in producing the upper arm in a single forging step. Therefore, conventionally, the upper arm has been produced as follows: firstly, a preform 61 as shown in FIG. 6 having a shape similar to that of a target product is produced through forging, and subsequently, the preform is subjected to a plurality of forging steps to thereby produce the upper arm 54 shown in FIG. 5.
Specifically, a solid round bar material 71 as shown in FIG. 7 is subjected to forging by use of a forging die to thereby yield a forged product having a flash 72 formed at its periphery. Subsequently, the flash 72 is removed from the forged product by use of a trimming die to thereby yield a preform 73. Thereafter, the preform 73 is subjected to a plurality of forging steps to thereby produce an upper arm 74. In this case, in order to reduce loss of the material incurred by formation of the flash 72, there is employed a forging die having a configuration allowing a plurality of forged products 73a to be produced from one solid round bar material 71 in a single step.
JP-A HEI 1-166842 discloses a method for producing, through closed forging, a product having a plurality of branches. In the prior art method for producing a product having a plurality of radially extending branches, as shown in FIG. 8, a punch 91 is used to apply pressure to a solid round bar material serving as a raw material so as to fill impressions provided in dies 93 and 94 to thereby form radially extending branches 92 through closed forging.
JP-A HEI 10-118735 also discloses a method for producing, through closed forging, a product having a plurality of branches. This method uses a casting-forging die that has a metal reservoir portion having a thickness larger than a modified surface layer of a forging material and that is provided between the inner surface of a die block, which inner surface is in parallel to a forging direction, and the outer surface of a punch. The casting-forging die also has a convex fringe portion provided on the outer surface of the punch for facilitating removal of a forged product. A cast material (forging material) is set in position inside the die block, and a forged product is formed through closed forging, with the modified surface layer of the forging material remaining in the metal reservoir portion.
Also, JP-A 2002-361354 discloses a method for producing, through closed forging, a product having a plurality of branches. This closed forging method employs, as a forging material, a cylindrical cast ingot comprising an upper surface and a lower surface each containing no angular portion and a circumferential surface, having the same volume as a preform and assuming a shape such that the ratio of the lateral length of a projection profile of the forging material to the length of the forging material as measured in the direction of pressure application is 1 or less, in which the profile is formed in a direction perpendicular to the direction of pressure application. In this method, pressure is applied onto the circumferential surface of the forging material to thereby produce a preform 15 of an upper arm or lower arm that is a suspension part for a vehicle.
The aforementioned conventional method for producing a preform of an upper arm or lower arm that is a suspension part for a vehicle requires a trimming step for removing flashes subsequent to a forging step. In this method, since unwanted flashes are formed on the preform, the yield of the preform on the basis of a forging material is low. In addition, since the projection area of the preform (i.e., forged product) as viewed in the direction of pressure application is large, a large, expensive forging machine capable of applying high load is required, resulting in high production cost.
In the closed forging method disclosed in JP-A HEI 1-166842, pressure is applied in a direction perpendicular to the cut surface of a cylindrical material so as to cause plastic flow of the material, thereby forming radially extending branches 92 (FIG. 8). Therefore, when the branches 92 are long or have different shapes, forging defects, such as underfill and overlap, on the surface of a forged product may be generated, because of differences in the rate or direction of plastic flow of the material between portions of the forged product.
In the closed forging method disclosed by JP-A HEI 10-118735, since the surface layer is extruded through tightly sealed forging, the load required for forging becomes large, resulting in possibly shortening the service life of the die. In addition, since restrictions are imposed on the balance matching in volume between the forging material and the forged product, there is a fair possibility of underfills being generated in the forged product due to balance mismatching.
Meanwhile, JP-A 2002-361354 does not disclose a specific step required for producing a target product from a preform although it discloses a method for forming the preform.
In view of the foregoing, objects of the present invention are to provide a forging method for producing a metal forged product having a plurality of branches, in which the yield of a target product on the basis of a raw material is improved; to provide a die employed in the forging method; and to provide a production system employing the die.
Another object of the present invention is to provide a method for producing a suspension part for vehicles and a preform of the part at low cost and in an efficient manner.
The term “material” as used herein refers to a product that has not yet been subjected to forging. The material encompasses cast ingot, forging material, cut material, solid round bar material, raw material, solid round bar, solid round bar raw material, cylindrical raw material, round bar material, continuously cast round bar, disk and billet material.
The term “preform” as used herein refers to a product which is obtained through forging and which requires one or more forging steps in order to be formed into a target product. The preform encompasses a blank, a rough forging blank and a rough blank.
The term “forged product” as used herein refers to a target product produced through forging. The forged product encompasses a member, a product, a final product, a forged final product and a final product produced through forging.